.odt is the default file extension for the upcoming OOo 2 Writer. It is only slightly different to the sxw format, and the main reason for the change in format mame / extension was to remove references to OOo or SO, so that the format can become a non program specific standard.

Of course OOo 2 will still be able to read/write .sxw files in the same way as OOo 1 can.